It is well known that a triarylamine compound is used as a charge transport material for an electrophotographic photoreceptor. For example, U.S. Pat. No. 3,180,730 discloses the use of triphenylamine and substituted products thereof as a charge transport material and U.S. Pat. No. 3,706,554 discloses the use of a tri-p-tolylamine compound. Also, JP-A-57-195254 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses the use of various triarylamine compounds in a charge transport layer of a laminated layer type electrophotographic photoreceptor and further, JP-A-61-132953 discloses an electrophotographic photoreceptor using a trisazo pigment and a triarylamine compound in combination. Also, U.S. Pat. No. 4,946,754 discloses elecrtrophotographic photoreceptors using a diarylbiphenylamine and various charge generating materials.
Also, recently triarylamine compounds having various substituents are disclosed, for example, in JP-A-1-280763, JP-A-2-36156, JP-A-3-282478, JP-A-3-78755, JP-3-78756,JP-A-78757, JP-A-2-230255, JP-A-2-190862, JP-A-2-190863, JP-A-2-78670, JP-A-2-178668, JP-A-2-178667, JP-A-2-178666, JP-A-2-156247, JP-A-2-134642, JP-A-4-193852, JP-A-4-133064, JP-A-4-118658, JP-A-3-285960, JP-A-3-249759, JP-3-223764, JP-A-3-114058, JP-A-3-127965, JP-A-3-101739, JP-A-3-102361, JP-A-3-58054, JP-A-3-56967, JP-A-3-7946, JP-A-3-7248, JP-A-3-5448, JP-A-3-2174, JP-A-2-243657, JP-A-2-178669, JP-A-3-213866, JP-A-3-163458, etc.
Most typical triarylamine compounds are triphenylamine and tri-p-tolylamine and they are effective as a charge transport material for an electrophotographic photoreceptor. However, since these compounds have a small molecular weight and a low melting point, when an electrophotographic photoreceptor using the compound in a charge transport layer is repeatedly used, there are disadvantages that the compound causes a migration in the charge transport layer and also is liable to cause bleeding, whereby the characteristics of the electrophotographic photoreceptor are changed with the passage of time.
For eliminating the above-described defects, it is considered to increase the molecular weight of the compound by introducing a substituent onto the compound. However, when a substituent not effective for transport of electrostatic charges is introduced, the distances between the molecules are increased to lower the mobility of electrostatic charges. The inventors found that an alkyl group is a substituent effective for transport of electrostatic charges, as described in Denshishashin Gakkai Shi (Journal of Electrophotographic Society), Vol.30, 16(1990). However, when an alkyl group having a longer chain than an ethyl group is introduced, the compound is not liable to stack due to steric hindrance of the alkyl group, rather lowering the melting point, so that the compound tends to cause migration and a stable electrophotographic photoreceptor is not liable to obtain.